CGSS 2023 Chemistry Prelim P2 Answers

1 Section A A1 (a) calcium chloride (b) ammonium chloride (c) hydrogen chloride (d) aluminium chloride (e) hydrogen chloride A2 (a) (i) The catalyst provides an alternative pathway of lower activation energy for the reaction to occur. More molecules/particles have energy greater than or equal to the activation energy. This leads to a higher frequency of effective collisions, leading to a greater rate of reaction. [1] role of a catalyst [1] explanation in terms of collisions and energy Discussion: • Reject: catalyst ‘lowers Ea’ more molecules have the minimum Ea (doesn’t cover the idea of greater than or equal to; Ea is an indication of energy, not energy itself) • Ignore: Greater frequency of collisions (ii) Catalysts are not chemically changed/regenerated at the end of the reaction so they can be reused infinitely. Hence, no additional cost is needed to purchase more new catalysts after each process. Catalysts provide an alternative pathway with lower activation energy for the reaction to occur, hence the reaction consumes less energy and lowers costs. [1] each - show an understanding of the workings of a catalyst: • regenerated at the end of the reaction, • requires less energy to operate since the reaction occurs with a lower Ea Note: student just needs to mention alternative pathway of lower Ea in (i) Discussion: • Reject: catalyst allows more hydrogen to be produced within a shorter period of time (does not relate to cost) unless they explain further about reduced manpower costs/labour costs only a small amount is needed (not about the quantity, but the fact that this small amount is regenerated and hence reused) (counter example: purchasing a catalyst requires a higher expenditure compared to when no catalyst is used) to include in Markers’ report: • Students need to write in a concise manner to avoid contradicting themselves in lengthy answers

(b) The reaction removes carbon monoxide/CO. [1] It is important to remove carbon monoxide , which is toxic/ poisonous/dangerous. I t reduces the ability of haemoglobin/red blood cell/blood to transport/carry oxygen to the rest of the body [1] [1] highlight what Stage 2 removes [1] impact of carbon monoxide on human health (don’t need to mention toxicity) Discussion: • mark allocation 1st mark: purpose of reaction 2nd mark: impact of CO on human health • allow: any other that CO causes reduced oxygen carrying ability (accounts for explain) (TB pg 394) eg. “CO reduces the amount of oxygen in the body” eg. “Lowers the amount of oxygen in the blood” • reject: no reference to oxygen carrying ability “CO can cause breathing problems and death in humans” (does not account for ‘explain’, but states the ‘effect’) Follow up: • check if ‘toxicity’ is required in the answer (2018/2/6b) Markers’ report implies that students need to elaborate on its toxicity Additional